How GeoExchange Systems Work?

The heat energy taken from theground by your GXS is consideredlow-grade heat. In other words, it is not warm enough to heat your home without being concentrated or upgraded somehow. However, there is plenty of it – the average temperature of the ground just a few metres below the surface is similar to (or even higher than) the average annual outdoor air temperature. For example, in Toronto, the average annual air temperature is about 8.9°C, but the average ground temperature is 10.1°C. It is important to note that this ground temperature is 10.1°C on the hottest day of summer as well as on the coldest day of winter. That is why some of the first humans lived in caves – the caves would protect them from the temperature extremes of winter and summer. That is also why a GXS works so efficiently – it uses a constant, relatively warm source (ground or water) from which to draw energy.

Basic Components of a GXS

Generally speaking, a GXS is made up of three main parts: a loop, the heat pump and the distribution system. The following section describes some of the various loop designs, heat pumps and distribution systems commonly used in a Canadian GXS.

The loop is built from polyethylene pipe which is buried in the ground outside your home either in a horizontal trench (horizontal loop) or through holes drilled in the earth (vertical loop). The loop may also be laid on the bottom of a nearby lake or pond (lake loop or pond loop). Your GXS circulates liquid (the heat transfer fluid) through the loop and to the heat pump located inside the home. The heat pump extracts heat from the ground and distributes the heat collected from it throughout the home. The chilled liquid is pumped back into the loop and, because it is colder than the ground, is able to draw more heat from the surrounding soil. These loops are often referred to collectively as a closed loop, as the same liquid circulates through the closed system over and over again.

Each of the ground-coupling systems already described utilizes an intermediate fluid to transfer heat between the ground and the refrigerant. Use of an intermediate heat-transfer fluid necessitates a higher compression ratio in the heat pump in order to achieve sufficient temperature differences in the heat-transfer chain (refrigerant to fluid to earth). Each also requires a pump to circulate water between the heat pump and the ground loop. Direct-expansion systems remove the need for an intermediate heat-transfer fluid, the fluid-refrigerant heat exchanger and the circulation pump. Copper coils are installed underground for a direct exchange of heat between refrigerant and ground. The result is improved heat-transfer characteristics and thermodynamic performance. However, the systems require a large amount of refrigerant and, because the ground is subject to larger temperature extremes from the direct expansion system, there are additional design considerations. In winter heating operation, the lower ground-coil temperature may cause the ground moisture to freeze. Expansion of the ice buildup may cause the ground to buckle. Also, because of the freezing potential, the ground coil should not be located near water lines. In the summer cooling operation, the higher coil temperatures may drive moisture from the soil.

Another way is to pump ground water or well water directly through the heat pump. A GXS that uses ground water is often referred to as an open-loop system. The heat pump extracts heat from the well water, which is usually returned to the ground in a return well. To run an open-loop GXS, you need two reliable wells with water that contains few dissolved minerals that can cause scale build-up or rust over the long term, as it is pumped through the heat pump’s heat exchanger.

In both cases, a pump circulates liquid through the loop and the heat pump. The heat pump chills (or collects the heat stored in) the liquid when it is being used as a source of heat, and circulates it back through the loop to pick up more heat. A system for a large home will require a larger heat pump and ground loop, with a circulation pump to match.

After the GXS has taken the heat energy from the ground loop and upgraded it to a temperature usable in your home, it delivers the heat evenly to all parts of the building through a distribution system. It can use either air or water to move the heat from the heat pump into the home. Forced air is the most common distribution system in most parts of Canada, although a hot-water or hydronic system can also be used.

Forced-Air Systems

A heat pump in a forced-air GXS uses a heat exchanger to take the heat energy from the refrigerant to heat the air that is blown over it. The air is directed through ducts to the different rooms in the home, as with any forcedair fossil fuel or electric furnace. The advantages of a forced-air GXS are as follows:

it can distribute fresh, outside air throughout the home if it’s coupled with a heat recovery air exchanger;

it can air-condition the home (by taking the heat from the air in your home and transferring it to the ground loop) as well as heat it; and

it can filter the air in your home as it circulates through the system.

A GXS is designed to raise the heat of the air flowing through the heat pump by between 10 and 15°C; fossil fuel or electric furnaces are designed to raise it by 20 to 30°C. That difference means a GXS must move more air through the home to distribute the same amount of heat as a conventional furnace. So to design an efficient, quiet forced-air GXS, the contractor designing the ductwork must take into account the larger amount of air to be moved.The ductwork should also have acoustic insulation installed inside the plenum and the first few metres of duct, as well as a flexible connection between the heat pump and the main duct to ensure quiet operation.

Hydronic (Hot-Water) Heating Systems

As we said earlier, a heat pump can heat either air or water. The latter type distributes the heat by means of a hydronic (or hot-water) heating system. If you choose it for your home, keep in mind that currently available heat pumps can heat water to no more than about 50°C.

This limits your choices for equipment to distribute the heat to your home. Hot-water baseboard radiators are designed to operate with water heated to at least 65 to 70°C; they are less effective when the water is not as warm. As a result, you will need larger radiators – or more of them – to distribute the same amount of heat. Or you can reduce the heat loss from your home by installing more insulation, so you need less heat.

You can also install radiant floor, or in-floor, heating systems. These are becoming more common because they can increase comfort and improve system efficiency. Again, you must make sure that your radiant floor heating system is designed to operate within the temperature capabilitiesof your GXS.

The temperature difference between the ground loop and the hot water distribution system depends on the efficiency of the GXS; the greater the difference, the less efficient the system. Typically, a GXS will extract heat from the earth at about 0°C. If a radiant floor heating system requires a temperature of 50°C to heat your home, the heat pump will produce about 2.5 units of heat for every unit of electricity used to operate the system. If the system can be designed to operate with water at 40°C, it will produce 3.1units of heat for every unit of electricity used to operate it. In other words, it will be about 25 percent more efficient.

Think about it this way – if you have hot spring water to heat your home, you do not need a heat pump. The hot spring is a totally free, 100 percent efficient source of energy. But if the temperature of the water from the well needs to be raised 5°C to be high enough to heat your home, you need some additional energy. If it has to be raised 20°C, you need even more energy. The greater the temperature difference, the greater the additional energy need.

If you are thinking of installing a radiant floor heating system in your home, you should tell the person designing it that you are planning to use a GXS. Make sure you take the following factors into account:

placing your floor pipe 20 cm (rather than 30 cm) apart reduces the water temperature required to heat your home by 4 to 5°C and increases the efficiency of your GXS by about 10 percent;

laying your floor heating pipe in concrete rather than using aluminum reflective plates with the pipe reduces the required temperature by 12 to 15°C, increasing the efficiency of your GXS by 25 to 30 percent;

suspending pipe in the joist space under a floor means that you will need temperatures higher than what your GXS can produce, unless the heat loss in the space is very low;

placing insulation under a slab-on-grade floor or under a basement floor reduces heat loss to the ground below; and

installing a control system that lowers the water temperature pumped through the floor as the outdoor temperature rises increases the efficiency of the GXS. This type of control is commonly called an outdoor reset control.